This invention relates generally to systems for electrically measuring certain chemical characteristics of fluids, e.g., concentration of certain analytes such as ions, gases and metabolites in human blood, and, more particularly, to electrical circuits for reducing the effects of electrical interference in such measurement systems.
Systems of this general kind can take the form of blood chemistry diagnostic systems integrated into
infusion fluid delivery systems of the kind commonly used in hospital patient care. Such fluid delivery systems infuse nutrients, medications and the like directly into the patient at a controlled rate and in precise quantities for maximum effectiveness. Infusion fluid delivery systems are connected to a patient at an intravenous (IV) port, in which a hollow needle/catheter combination is inserted into a blood vessel of the patient and thereafter an infusion fluid is introduced into the vessel at a controlled rate, typically using a peristaltic pump. Blood chemistry monitoring systems that are combined with infusion delivery systems of this kind use the IV port to periodically withdraw a blood sample, perform measurements of blood ion concentrations and the like, and then discard the blood or reinfuse it into the patient. The system then resumes delivery of the infusion fluid.
Such combined infusion fluid delivery and blood chemistry monitoring systems include an infusion line and catheter through which the infusion fluid is provided to the patient and blood samples are withdrawn. The infusion line incorporates an electrode assembly having electrochemical sensors that are periodically exposed to the blood samples and thereby provide electrical signals to an analyzer for conversion into corresponding blood chemistry data. A control unit periodically halts delivery of the infusion fluid for a brief interval, during which time a blood sample is withdrawn from the patient into the infusion line and routed to the electrode assembly, which then generates the electrical signals. After the electrical signals have been received by the analyzer, the control unit disposes of the blood or reinfuses it into the patient, and the flow of infusion fluid is resumed.
The electrode assembly typically includes a reference electrode and a plurality of sensor electrodes that are each sensitive to a particular ion of interest. All of the electrodes are embedded in the base of the electrode assembly. Electrochemical sensors generate electrical signals, either a voltage or a current, only in response to contact with the particular species to which they are sensitive, and therefore provide selective measurement of the amount of that species in the blood. Sensor electrodes can be provided to measure, for example, partial pressure of oxygen (pO.sub.2) and carbon dioxide (pCO.sub.2), glucose, calcium, hydrogen ion, chloride, potassium, and sodium.
The accuracy of the measurements described above can be adversely affected by any electrical current interference, usually originating at the patient, that is conducted along the infusion tube by the blood and the infusion fluid. Appropriate low-pass filtering of the electrical potential measurements can reduce the effects of this noise; however, substantial inaccuracies remain. Movement of the infusion tube relative to the patient provides even greater noise and makes the task of filtering or otherwise reducing the effects of the noise even more difficult.
It should therefore be appreciated that there is a need for an electrochemical measurement system of this particular kind that is less susceptible to interference from electrical current noise being conducted along the fluid line. The present invention fulfills this need.